1. Field of the Invention
The present invention relates to titanium carbide/tungsten boride coatings having excellent corrosion and wear resistance and to a process for preparing such coatings. More particularly, the invention relates to hard, dense, low-porosity, corrosion and wear resistant coatings containing ultrafine particles of titanium carbide and tungsten boride precipitates dispersed in a metallic matrix. The invention also relates to a process for preparing such coatings in situ by thermal spray and diffusion reaction techniques.
Throughout the specification, reference will be made to plasma arc spraying and detonation gun (D-Gun) techniques for depositing coatings. Typical detonation gun techniques are disclosed in U.S. Pat. Nos. 2,714,563 and 2,950,867. Plasma arc spray techniques are disclosed in U.S. Pat. Nos. 2,858,411 and 3,016,447. Other thermal spray techniques are also known, for example, so-called "high velocity" plasma and "hypersonic" combustion spray processes, as well as the various flame spray processes. Heat treatment of the coatings is necessary and may be done after deposition in a vacuum or inert gas furnace or by electron beam, laser beam, induction heating, transferred plasma arc or other technique. Alternative deposition techniques such as slurries, filled fabrics or electrophoresis, followed by heat treatment, are also known. Still other methods include simultaneous deposition and fusion utilizing plasma transferred arc, laser or electron beam surface fusion with or without post deposition heat treatment.
2. Background Art
Cutting tools are usually made of tungsten carbide-cobalt alloys. These alloys are extremely hard, strong and tough and exhibit excellent wear properties under most conditions of use. However, a problem with these alloys has been that tungsten carbide is subject to oxidation at temperatures above about 540.degree. C. When operated at these elevated temperatures for any sustained period, cutting tools made of these alloys loose their wear properties and frequently crack, spall or chip.
Chemical vapor deposition techniques have been used to improve the wear properties and oxidation resistance of tungsten carbide-cobalt cutting tools by depositing a thin layer of titanium diboride (TiB.sub.2) on the surface of the parts. Due to interactions between TiB.sub.2 and WC/Co at elevated temperatures, a thin film which is less than about 30 microns thick, is formed on the surface of the cutting tools which contains CoWB and TiC compounds. Titanium carbide has a higher oxidation resistance than tungsten carbide and is more stable. Consequently, the formation of a film containing these compounds increases the wear resistance of the cutting tools.
Vapor deposited films containing CoWB and TiC are furthermore limited to use with only a few substrates, particularly tungsten carbide-cobalt alloys. It would be advantageous therefore to develop TiC/WCoB coatings which can be applied to a variety of substrate materials.